this is a zip of all the info that came with A Qkits amplifier kit I got once ampon a time that seems to be discontinued now. Anyways, it is rates 100x1@8ohm or 175x1@4ohm and i verified it at smoke X lots @ 2ohm. I am going to replce the fried 3055's with MJ15003's as it was reccomended to me on this board as being a better all around transistor. This schematic and parts list might help you get started tho..

On many versions of the ST-120 that were kit built, the output's were indeed 2n3055. Reviewing the parts list of my ST-120 Q5/Q6 were 2N3055 Dynaco Part # 571844. (Serial number 17828060).
Another test to see what version you have, look at Q3/Q4 (drivers). If they are 2n3053/2n4037 and NOT the TIP's, then you have a older kit version of the ST-120 that has not been modified.

On later models they did switch to a better output/drivers.

The reason a 2N3055 works is due to the regulated 72V power-supply. That's right the ST-120 actually had a regulated supply.
You may ask, but wait the 2N3055 has too low a Vceo. That is true but it does have a Vcer of 70 volts and a Vcb of 100V. Thus allowing a 2N3055 to just squeek in, assuming a little power supply droop.

The regulated supply is what I feel gives this amplifier it's unique sound even with slow outdated outputs such as 2N3055's.

Well, for what it's worth, I just switched an ESP3A amp from 2N3055/MJ2955 to the MJ15003/15004. There was a significant difference. It sounded pretty good before, but sounded far better afterwards.
I have some spare old 2N3055s if you wanna drop by to pick 'em up...

Originally posted by ted On many versions of the ST-120 that were kit built, the output's were indeed 2n3055. Reviewing the parts list of my ST-120 Q5/Q6 were 2N3055 Dynaco Part # 571844. (Serial number 17828060).
Another test to see what version you have, look at Q3/Q4 (drivers). If they are 2n3053/2n4037 and NOT the TIP's, then you have a older kit version of the ST-120 that has not been modified.